In industrial fluid control, the valve is the final executing element of any control loop. But even the most sophisticated PLC or PID controller cannot deliver stable process control if the valve’s flow characteristic fights the system dynamics. The flow characteristic curve—a graph plotting relative flow against relative stem travel—is the single most important document for predicting how a control valve will behave in your system. At Dofun, with more than 30 invention patents and a product portfolio spanning Pressure Regulating Valve Series, Motor Valve Series, Solenoid Valve Series, and Valve Station Series for refrigeration, petrochemical, energy, food, pharmaceutical, and defense applications, we understand that helping our clients read and interpret these curves is as critical as the valve itself. This article explains how to decode equal percentage and linear flow characteristic curves, and why the right choice directly determines your system’s control stability, energy efficiency, and long-term reliability.

What Is a Valve Flow Characteristic Curve?

A valve flow characteristic describes the mathematical relationship between the valve stem position (relative opening) and the flow rate passing through the valve. Graphically, it is plotted with relative flow (Q/Qmax) on the vertical axis and relative stem travel (L/Lmax) on the horizontal axis.

There are two distinct types of characteristics every engineer must understand:

Inherent Flow Characteristic

This is the idealized curve measured under laboratory conditions where the pressure drop (ΔP) across the valve is held constant. It is determined purely by the geometric shape of the valve plug and seat. Manufacturers publish this curve in product datasheets.

Installed Flow Characteristic

This is the actual performance when the valve operates in a real piping system. As flow increases, friction in pipes, coils, and fittings consumes a larger portion of the available pressure, reducing the ΔP across the valve. The result is that the installed curve deviates significantly from the inherent curve.

Understanding the distinction is critical: you select based on inherent characteristic, but you are actually controlling based on installed characteristic.

Linear Flow Characteristic: Constant Gain, Constant Sensitivity

The Mathematics and Shape

In a linear characteristic, flow is directly proportional to stem position. The relationship follows a simple linear equation:

Q/Qmax = L/Lmax

On a graph, this appears as a straight diagonal line. At 50% opening, the valve passes 50% of maximum flow.

Control Behavior

The defining feature of a linear valve is that its gain (Δflow/Δtravel) remains constant across the entire stroke. A 10% increase in opening always produces a 10% increase in flow, regardless of the starting position.

Ideal Application Conditions

Linear characteristic delivers optimal control when:

• Pressure drop across the valve remains constant (or nearly so) regardless of flow rate

• System load varies little (load change ratio < 3:1)

• Piping resistance is low (short pipe runs, minimal fittings)

Typical applications include:

• Liquid level control in atmospheric tanks

• Cooling water control with constant supply pressure

• Boiler feedwater control with steady pump head

• Constant-pressure bypass regulation

The Limitation in Real Systems

Here is where linear valves encounter trouble. In most real-world systems, as the valve opens and flow increases, pipe friction increases with the square of the flow rate, consuming a larger share of the available pressure. This leaves less ΔP for the valve. The consequence: a linear valve installed in a typical system exhibits equal-percentage-like behavior, with severe distortion at high flows.

As documented in Spirax Sarco’s control valve analysis, an oversized linear valve can become “fast-acting” for up to 90% of its range, making stable control extremely difficult.

Equal Percentage Flow Characteristic: The Workhorse of Process Control

The Mathematics and Shape

In an equal percentage characteristic, each equal increment of stem travel produces a percentage change in flow proportional to the flow at the start of that increment. The relationship follows:

Q/Qmax = R^(L/Lmax − 1)

Where R is the rangeability ratio (Qmax/Qmin), typically 25 to 50 for control valves.

On a graph, this appears as an exponential curve—relatively flat at low openings, steepening dramatically at high openings.

Control Behavior

The defining feature is that gain increases proportionally with flow. At low flows, the valve provides fine, precise control (low gain). At high flows, the valve delivers aggressive response (high gain).

Why Equal Percentage Dominates

Equal percentage is the default choice for 70–80% of process control applications because it compensates for the natural pressure drop variation in real systems. As the valve opens:

1. Pipe friction increases, reducing ΔP available to the valve

2. The valve’s inherent gain naturally increases with opening

3. These two effects cancel each other out

4. The result: the installed characteristic becomes nearly linear, providing stable control loop gain across the entire operating range

Ideal Application Conditions

Equal percentage characteristic excels when:

• Pressure drop across the valve varies with flow (typical in most real systems)

• System load varies significantly (load change ratio > 3:1)

• Piping resistance is high (long pipe runs, multiple fittings, coils)

• Process itself is non-linear (e.g., heat exchangers where heat transfer efficiency changes with flow)

Typical applications include:

• Temperature control in heat exchangers, condensers, evaporators

• Pressure control in varying flow conditions

• Steam systems where load fluctuates

• Refrigeration systems with variable thermal loads

Side-by-Side Comparison: Linear vs. Equal Percentage

Parameter	Linear	Equal Percentage
Curve shape	Straight diagonal line	Exponential curve
Gain behavior	Constant across stroke	Increases with flow
Small opening (10–20%)	Flow changes rapidly (can cause overshoot)	Flow changes slowly (fine control, anti-overshoot)
Large opening (80–90%)	Flow changes slowly (“can’t add enough”)	Flow changes rapidly (full capacity available)
Rangeability	Medium (typically 10:1 to 20:1)	High (30:1 to 50:1)
Installed behavior in real systems	Distorts toward quick-opening	Compensates toward linear
Best for	Constant ΔP, low load variation	Varying ΔP, high load variation
Typical uses	Level control, constant-pressure systems	Temperature, pressure, refrigeration control

The Critical Role of Valve Authority

The degree to which installed characteristic matches inherent characteristic is quantified by valve authority (N):

N = ΔP_valve,rated / (ΔP_valve,rated + ΔPsystem,rated)

Where:

• ΔP_valve,rated = valve pressure drop at design flow with valve fully open

• ΔPsystem,rated = pressure drop through all other system components at design flow

Interpretation:

• N > 0.5: High authority; installed characteristic closely matches inherent characteristic

• N = 0.25–0.5: Moderate authority; some distortion, manageable

• N < 0.25: Low authority; severe distortion, marginal control at high flows

Key insight: Equal percentage valves require less authority than linear valves to maintain acceptable installed characteristics. This is why equal percentage is more forgiving in real-world installations where achieving high valve authority is economically or physically challenging.

For a linear valve with authority N = 0.5, the installed flow at 50% valve position is 67% of maximum rather than the inherent 50%. At authority N = 0.25, installed flow jumps to 80%—demonstrating severe compression of the control range.

Impact on System Control: Stability, Energy, and Reliability

The choice between linear and equal percentage is not academic—it directly affects three critical operational outcomes:

1. Control Loop Stability

An improperly matched flow characteristic creates variable loop gain. When gain is too high at certain operating points, the loop oscillates (“hunts”). When gain is too low, the loop responds sluggishly. Equal percentage valves, by compensating for ΔP variation, maintain more consistent loop gain—resulting in stable control with minimal tuning effort.

2. Energy Efficiency

In refrigeration and HVAC systems, control valve performance directly impacts compressor and pump energy consumption. A valve that provides precise, stable control allows the system to operate closer to optimal setpoints without over-correction. Dofun’s large-scale cold chain clients have confirmed that our control valves *”achieve exceptional precision, playing an instrumental role in ensuring our cold chain safety and energy efficiency management.”* This precision is fundamentally enabled by correct flow characteristic selection.

3. Valve and Trim Longevity

When a control loop hunts due to mismatched flow characteristic, the valve actuator cycles excessively, packing wears prematurely, and trim erodes. Selecting the correct characteristic—typically equal percentage for varying ΔP systems—dramatically extends service life. As validated by our mining sector clients: *”The solution provided by Dofun, with its remarkable durability, has significantly reduced our maintenance frequency and spare parts costs.”*

Dofun’s Flow Control Portfolio: Engineered for the Right Characteristic

Dofun manufactures a comprehensive range of valves for precise flow control across diverse applications. Our product lines include:

Pressure Regulating Valve Series

Our IPRV (Inlet Pressure Regulating Valve), OPRV (Outlet Pressure Regulating Valve), and DPAV (Differential Pressure Regulating Valve) products, rated for NH₃, Freon, and CO₂ systems across DN25–DN125 with operating temperatures from −50°C to +150°C, are engineered to maintain stable downstream or upstream pressure regardless of flow variation. These self-operated regulators embody equal-percentage principles in their pilot-controlled trim design, automatically compensating for system pressure fluctuations.

Motor Valve Series (DFMV)

Rated at PN40/PN63 with temperature range −50°C to +150°C, our motor-driven control valves provide precise modulation for large industrial refrigeration and process applications. Available in DN20–DN125 sizes, these valves are designed for applications where equal percentage trim characteristics deliver optimal control stability.

Solenoid Valve Series (DEVSC/DEVS)

For automated on/off and piloting functions, our solenoid valves (DN3–DN50, applicable to R12, R22, R502, R134a, R404A, R507, R717, CO₂, and other refrigerants) provide reliable, fast-response control. While solenoid valves are primarily on/off devices, they serve as critical pilot valves in our pressure regulating systems, where their rapid response enables the equal-percentage compensation effect.

Valve Station Series (DFCV)

Our integrated valve stations (rated 4.0 MPa, −50°C to +150°C) consolidate multiple control functions—stop valves, check valves, filters, regulators—into a single manifold. Built on the DOFUN-CTP modular shared platform, these stations allow engineers to specify the optimal flow characteristic for each control function while maintaining installation simplicity.

All Dofun control valves are manufactured in our modern 30-acre facility in Liaocheng, equipped with globally advanced production lines and testing platforms, ensuring that the flow characteristic designed into each valve trim is precisely delivered in the finished product.

How to Select the Right Flow Characteristic: A Practical Framework

When specifying Dofun control valves for your application, we recommend this structured evaluation:

Step 1: Assess System Pressure Drop Behavior

• Is ΔP across the valve constant? → Consider linear

• Does ΔP vary significantly with flow? → Equal percentage (almost certainly)

Step 2: Evaluate Load Variation

• Load change ratio < 3:1 (stable process)? → Linear may suffice

• Load change ratio > 3:1 (fluctuating process)? → Equal percentage

Step 3: Analyze Piping System

• Short pipe runs, low fitting count → Inherent ≈ installed; linear viable

• Long pipe runs, many fittings, coils → Significant distortion; equal percentage required

Step 4: Define Process Dynamics

• Level control (linear process gain): Linear characteristic

• Temperature/pressure control (non-linear process gain): Equal percentage

Step 5: Calculate Valve Authority

• Target N > 0.5 for linear valves (difficult to achieve economically)

• Equal percentage valves perform acceptably even at N = 0.25–0.5

Step 6: Consult Dofun Engineering

Our technical team will review your specific operating conditions—including medium (NH₃, Freon, CO₂, etc.), temperature range, pressure profile, flow variation, and control objective—to recommend the optimal valve type and trim characteristic. Leveraging our 30+ invention patents and deep application experience across refrigeration, petrochemical, energy, food, pharmaceutical, and defense sectors, we ensure the selected valve delivers stable, efficient, and reliable control.

Real-World Example: Refrigeration System Evaporator Pressure Control

Consider a typical industrial refrigeration system where an Outlet Pressure Regulating Valve (OPRV) controls evaporator pressure to maintain stable suction conditions. The system characteristics:

• Load varies from 30% to 100% of design capacity (seasonal and diurnal variation)

• Piping resistance is significant (long suction lines, multiple fittings, evaporator coils)

• Compressor curve means ΔP across the valve varies with flow

• Control objective: stable evaporator pressure for consistent heat transfer

In this scenario:

• A linear valve would exhibit severe distortion—providing excessive sensitivity at low loads (causing pressure hunting) and insufficient authority at high loads (failing to maintain setpoint)

• An equal percentage valve compensates for the ΔP variation, maintaining consistent loop gain and stable pressure control across the entire load range

Dofun’s OPRV and IPRV products are specifically designed with equal-percentage compensation principles, making them the optimal choice for such refrigeration applications. Our clients in large-scale cold chain operations have confirmed that these valves *”achieve exceptional precision, playing an instrumental role in ensuring our cold chain safety and energy efficiency management.”*

Conclusion: The Curve That Determines Everything

Reading a valve flow characteristic curve is not merely an academic exercise—it is the foundation of stable, efficient, and reliable process control. The key takeaways for engineers and specifiers are:

1. Linear characteristic provides constant gain and suits systems with constant ΔP and low load variation

2. Equal percentage characteristic compensates for real-world ΔP variation, delivering stable control in the vast majority of process applications

3. Installed characteristic differs from inherent characteristic—valve authority quantifies this deviation

4. Equal percentage valves require lower authority to maintain acceptable control, making them more forgiving in real installations

5. Correct selection impacts stability, energy efficiency, and equipment longevity

With products widely used across refrigeration, petrochemical, refining, nuclear power, chemical, metallurgical, mining, medicine, food, and national defense, and recognized as a national-level specialized and innovative enterprise, Dofun does not simply manufacture valves—we engineer flow control solutions. Our Pressure Regulating Valve Series, Motor Valve Series, and Valve Station Series are designed with deep understanding of flow characteristic physics, ensuring that the curve on the datasheet translates into stable performance in your plant.

When specifying control valves for your next project, do not leave the flow characteristic to chance. Contact Dofun’s technical team to discuss your specific operating conditions, load profiles, and control objectives. Together, we will select the optimal valve with the optimal flow characteristic—equal percentage, linear, or a modified characteristic tailored to your process—ensuring your control loop delivers the stability, efficiency, and reliability your operation demands.

Dofun’s Engineering Principle: A valve is only as good as its match to the system. By understanding flow characteristic curves, we transform a simple mechanical component into a precision instrument of process control—carving every Dofun product into a metal artwork that performs flawlessly, cycle after cycle, under the most demanding conditions.